The present invention relates to a process for producing a microporous membrane of ultra-high-molecular-weight polyolefin, especially ultra-high-molecular-weight polyethylene. More particularly, it is concerned with a process for efficiently producing a uniform and relatively thick microporous membrane of an ultra-high-molecular-weight polyolefin.
Microporous membranes are widely used in various applications such as battery separators, electrolytic capacitor separators, various filters, moisture-permeable, waterproof clothes, reverse osmosis membranes, ultrafilters, microfilters, etc.
Microporous polyolefin membranes are conventionally produced by various processes. One example of such processes is an extraction process comprising the steps of mixing a polyolefin with a pore-forming agent such as a fine powder or a different polymer in such a manner as to achieve micro-dispersion and subsequently extracting the dispersed pore-forming agent. Another process is a phase separation process by which polyolefin is divided into fine phases by a solvent, thereby forming a porous structure. Further, there is a stretching process comprising the steps of forming a polyolefin article containing different solid finely dispersed therein and imparting a strain to the article by stretching to break the interfaces between the polymer phase and the different solids, thereby forming pores in the article. In these processes, however, polyolefins having a molecular weight lower than 500,000 are usually used, so that the thinning and strengthening of membranes by stretching are limited.
Recently, there has been developed a ultra-high-molecular-weight polyolefin which can be formed into a high-strength, high-modulus fiber or film. With this development, there have been proposed various processes for producing high-strength and high-modulus fibers or films from it.
One of such process is disclosed in, for example, Japanese Patent Laid-Open No. 58-5228. According to this process, an ultra-high-molecular-weight polyolefin is dissolved in a nonvolatile solvent and the resulting solution is made into a gel in the form of fiber or sheet. The solvent-containing gel or dry gel from which the solvent has been extracted with a volatile solvent is stretched while heating. This process is, however, disadvantageous in that the solvent containing gel cannot be biaxially stretched at a high draw ratio because it has a porous structure highly swollen with a nonvolatile solvent. The resulting membrane has a low strength and a large pore diameter on account of its reticulate structure which easily expands and breaks. Another disadvantage of this process is that the gel extracted with a volatile solvent in the form of a sheet is liable to warpage because of uneven evaporation of the volatile solvent. Further, it cannot be subjected to stretching at a high draw ratio because of the shrinkage and compaction of the reticulate structure of the gel which take place after the extraction of the nonvolatile solvent by a volatile solvent.
Japanese Patent Laid-Open No. 60-242035 discloses a process for producing a microporous ultra-high-molecular-weight polyethylene membrane having a thickness of 10 .mu.m or less, a breaking strength of 200 kg/cm.sup.2 or more, and a void volume of 30% or more by dissolving ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 5.times.10.sup.5 or more in a solvent while heating, forming a gel-like sheet from the resulting solution, removing a solvent from the gel-like sheet until the solvent content decreases to 10-80 wt %, and then stretching the sheet while heating, thereby removing a residual solvent.
According to this process, the gel-like sheet is usually about 0.1-5 mm thick, and it is produced from a solution containing 1-10 wt % of polyethylene. After the removal of solvent, the gel-like sheet is biaxially stretched such that the linear draw ratio is twofold or more and the areal draw ratio is tenfold or more.
Unfortunately, being as thin as 10 .mu.m or less, the microporous ultra-high-molecular-weight polyethylene membrane thus produced is not suitable for use in applications where high strength and high pressure resistance are required. Any attempt to produce a thicker membrane by increasing an amount of a polymer solution cast on a substrate, however, inevitably leads to a reduced cooling rate which promotes crystallization of the polymer to such an extent that stretching is not easily achieved. Further, in the case where the gel-like sheet is formed by extrusion of an ultra-high-molecular-weight polyethylene solution from a die, any attempt to increase a thickness of the extruded sheet fails to provide a uniform sheet because of excessive swelling and neck-in. In addition, the thicker a sheet, the more slowly it is cooled. Accordingly, excessive crystallization takes place, resulting in a difficult-to-orient molecular structure.